The current interest in avoidance of air pollution has in a sense only emphasized an ongoing trend to prevent massive discharge of particulate matter, often of commercial value, into the ambient atmosphere. Air filters of many types are known, more or less specially adapted for widely varying uses.
There are numerous situations where a large volume of a fluid such as air laden with particulate matter must be treated on a continuous basis at a predetermined rate in terms of volume per unit time. In order to handle a large volume per unit of time, i.e. several thousand cubic feet per minute, a large filter area must be available, and even then a very considerable pressure drop takes place across the filter. From a practical viewpoint it is necessary that some form of continuous cleaning of the filter medium be provided, as duplication of equipment to permit shutdown for cleaning is prohibitively expensive both in cost and in space, while the period of operation of a filter unit before its efficiency is reduced to an intolerable level by the matter accumulated thereon is not long.
A standard way of constructing air filters has been to provide a housing divided into inlet and clean air chambers by a partition having a plurality of apertures. A sock or bag of suitable fabric, often on a wire frame, is suspended in each aperture, and a suitable pump or blower causes air flow through the bags, so that the undesired particulate matter collects on the outer surfaces of the bags. Cleaning is accomplished by directing brief jets of air into the bags, a few at a time, in a predetermined sequence, thus reversing the flow through the bags and subjecting them to a minor physical shaking. This removes the deposited material, most of which falls to the bottom of the inlet chamber for removal either intermittently or continuously with a rotary air lock.
In a typical example, a filter of this sort to handle approximately 8000 cubic feet of air per minute with a flow velocity of 8-10 cubic feet per minute per square foot of area of medium (which corresponds to an air flow velocity through the filter media of 8-10 feet per minute), requires 770 square feet of bag surface, a total of 81 bags 41/2 inches in diameter and 8 feet long. Thus even apart from the continuous cleaning components, the filter elements alone occupy a housing of very considerable size.
It also became obvious very quickly that the filter bags and their supporting frames, are bulky and awkward to deal with, difficult to install and replace, and subject to rapid wear.
Efforts have been made to find substitutes for filter bags, and some of these alternatives have been successful to a limited extent in laboratory situations. Thus porous metal and ceramic media turned out to have too low a capacity, and woven metal media are prohibitively expensive. Pleated filters made of paper gave some promise, but had too short a lifetime to be practical.